Pumps for feeding fuels are known in the practice in different designs. These pumps are generally installed on the fuel tank, which typically is at the rear or side of the vehicle, or close to the tank at the upstream tank end of the line that runs to the engine, typically at the front of the vehicle, where there is another pump forming part of the fuel-injection system of the internal-combustion engine. During operation of the internal-combustion engine the motor-driven fuel feed pump supplies the injection system with fuel.
When, however, the fuel tank is run completely empty, the fuel feed pump cannot bleed the fully drained fuel line after the tanks have been filled. To avoid damage to the fuel feed pump the fuel line is in most cases bled with a manually operated priming pump. To this end the tank-side fuel line is connected to the inlet and the engine-side fuel line to the outlet. To prevent the fuel from flowing back into the tank when bleeding during the pumping process, both the inlet and the outlet are provided with valves. Different pumps that are fitted with different types of valves are known from the state of the art. Fuel must also flow through the valves during normal operation of the internal-combustion engine, resulting in major pressure losses during the fuel feed process. This also means that the outlet of the fuel feed pump must be designed so that it is clearly over-dimensioned, making its manufacture expensive and costly. Because of the high pressure loss the pumps of prior art cannot satisfactorily meet the requirements that are imposed by diesel engines operated with high pressure injection systems in particular.
A further disadvantage of the known priming pumps practice consists in the fact that due to the stiffness of the pump piston the need to refill the fuel line is often difficult to detect since only the resistance of the piston during the pumping process is available as an indicator of the bleeding process that has taken place. Furthermore, the prior-art pumps art can only be installed in one prescribed alignment. Because of this it is often only possible at considerable cost to integrate such a pump in a fuel supply system so that both the alignment of the pump complies with the specifications of the manufacturer and perfect operability is guaranteed.